GEOLOGY OF CENTRAL WALES. 



140 



great series of thin-bedded, slate, pale- or dark-coloured, seen in the 

 ascent from this bridge to the top of the high part of Rhyader, 

 appears to show these rocks as in regular serial continuity with 

 the beds passed over on the great slope next to the west, the whole 

 forming the vast westerly fold complementary to the great easterly 

 dip of the Metalliferous-slate series between Aberystwyth and the 

 Devil's Bridge. And just as the latter is now proved to be not one 

 continuous series, but really a much smaller group affected by fre- 

 quent inversion, so we are, I think, justified in adopting a like inter- 

 pretation for these beds to the east of the Plynlimmon axis. The 

 correspondence of the rocks on the two meridional sides of this axis 

 is further illustrated as we reach the eastern edge of the great plateau 

 above Gwyn Llyn (fig. 2), where we find a zone of pale banded and 

 indurated slate rock identical with the Strata Florida slate rock used 

 in the Aberystwyth stone pier. These pass under a series of grits 

 and conglomerates in the beautiful valley of Cwm Elan. A little 

 further south these latter beds reappear in the Grwastaden grits 

 and conglomerates of the hills east of Rhyader. Thus our accumu- 

 lated facts support the view that there are two distinct series of 

 grits in Cardiganshire, separated by the Metalliferous-slate group. 

 The Ehyader grits and conglomerates appear to be nearer to the 

 Plynlimmon than the Aberystwyth group. 



The Aberystywth Grits, extent, variations. Sfc. — -A glance at the 

 yellow-dotted area upon the Geological Survey Map will show that 

 the Aberystwyth grits form a crescent-shaped patch of country in 

 the centre of the western Welsh border, extending from near Borth, 

 some five miles north of Aberystwyth, to Traeth Bach, south of 

 Llangrannog, a distance of thirty miles, and with a maximum 

 breadth, at Mynydd Bach, of nine miles. 



A set of characteristic surface-features marks off its boundaries 

 with tolerable distinctness, its barren or gorse-covered ridgy hills, 

 elongated along the line of rock strike, and with minor sets of 

 parallel crests and ledges (sometimes forming sets of small step- 

 like structures or inclined ribs in the lines of more durable grit-beds), 

 giving to the group an almost unmistakable appearance*. 



The general characters of the rocks have already been described 

 in our Devil's-Briclge line of section ; and the variations from those 

 types are neither numerous nor very striking. The greywacke and 

 grits are very constant in grain, never becoming conglomeratic ; but 

 some coarser varieties are found in a few localities — for example, 

 around the Eiddwen lakes, Mynydd Bach, and at Pen Craig, north of 

 Llanilar. In these places the greywacke is almost granitoid in 

 appearance, the quartz grains being large, flakes of mica common, 



* It was pointed out long ago by Sir Henry de la Beche that the same type 

 of physical features appears in the area of the grits of Peurhyn ddu in the 

 Lleyn peninsula, which are marked Lower Cambrian upon the Survey Map. 



To me this resemblance also appeared very striking. The Penrhyn-ddu beds 

 are totally unlike any of the Longmynd group known to me; and I cannot 

 believe they are of such an age ; nor do I think with de la Beche that they are 

 of the same age as the Aberystwyth beds, but regard them as an exceptionally 

 developed type of the Tremadoc series. 



Q.J.G.S. No. 146. m 



